add failing test for erroneous apostrophed-encoding

src/mimeparser.rs

@@ -1604,6 +1604,17 @@ mod tests {
         assert_eq!(filename, Some("Auftragsbestätigung.pdf".to_string()))
     }
 
+    #[async_std::test]
+    async fn test_get_attachment_filename_apostrophed_invalid() {
+        let t = TestContext::new().await;
+        let mail = load_mail_with_attachment(
+            &t,
+            include_bytes!("../test-data/message/attach_filename_apostrophed_invalid.eml"),
+        );
+        let filename = get_attachment_filename(&t.ctx, &mail.subparts[1]).unwrap();
+        assert_eq!(filename, Some("somedäüta.html.zip".to_string()))
+    }
+
     #[async_std::test]
     async fn test_get_attachment_filename_combined() {
         // test that if `filename` and `filename*0` are given, the filename is not doubled

test-data/message/attach_filename_apostrophed_invalid.eml

@@ -0,0 +1,30 @@
+Subject: Test apostrophed filenames
+Message-ID: 12345@testrun.org
+Date: Sat, 07 Dec 2019 19:00:27 +0000
+To: recp@testrun.org
+From: sender@testrun.org
+Content-Type: multipart/mixed; boundary="==BREAK=="
+
+
+--==BREAK==
+Content-Type: text/plain; charset=utf-8
+
+this mail uses filename* in an invalid way,
+not specifying chatset or language parts.
+
+see https://tools.ietf.org/html/rfc2231, section 4.:
+"Note also that the single quote delimiters MUST be present
+even when one of the field values is omitted."
+
+however, for now, Delta Chat is generous in this case,
+just assuming encoded words
+
+--==BREAK==
+Content-Type: text/html
+Content-Disposition: attachment;
+ filename*="=?utf-8?Q?somed=C3=A4=C3=BCta.html.zip?="
+Content-Transfer-Encoding: base64
+
+PGh0bWw+PGJvZHk+dGV4dDwvYm9keT5kYXRh
+
+--==BREAK==--